If anyone knows anything about outdoor flea prevention/beneficial nematodes please open me!!!

So we have a large squirrel family living in 2 huge maple trees at the back of my yard. Because of this we have been battling fleas for most of the summer.

We just moved here last year so this is our first summer here, we usually use diatomaceous earth/apple cider vinegar for flea prevention but this year it wasn't quite enough.

We have it pretty much under control now but would like to put something outside to help prevent it from happening again next year. We would like to avoid any chemical prevention/treatment on our pets so revolution isn't the answer I'm looking for right now.

Ive heard beneficial nematodes can do wonders but I'm not sure what brand to buy or really what to do with it? Can they be put in your lawn in the fall?

Any advice to help this problem would be appreciated!!

* please no negativity